8-CELL LION BATT TP X40 SERIES Stay unplugged longer. Get twice the capacity of the 4-Cell system battery and up to 7.5 hours battery life on select systems. This high capacity battery has an extended area on the back edge and fits in the standard ThinkPad X40 Series system battery slot. Its design allows it to be attached to the ThinkPad when it is connected to the optional ThinkPad X4 UltraBase Dock. All-day computing is a reality (up to 10.7 hours) on select systems when the ThinkPad X40 Series 8-Cell Li-Ion Battery is combined with the optional ThinkPad Ultrabay Slim Li-Polymer Battery in the ThinkPad X4 UltraBase Dock.